In the oil industry, "slips" have been necessary elements of oil field drilling equipment for many years. Classic slips are sets of heavy hinged blocks with gripping dies that are positioned in a slip bowl of a rotary table to engage drill pipe or casing. Angled surfaces in each slip block mate with angled surfaces in the slip bowl. The angled surfaces cause axial forces exerted by the pipe on the blocks to be transferred into lateral gripping pressure on the pipe to support the pipe and thus prevent it from slipping through the slips.
As is well known in the art, classic slips are engaged by oilfield personnel called "roughnecks" who physically maneuver the heavy slips into the slip bowl so that they slide into engagement with the casing or drill pipe. The slips are disengaged by upward axial movement of the casing or drill pipe to take the weight of the pipe off the slips. The slips are then lifted out of the slip bowl to disengage them from the casing or drill pipe and permit their removal from the slip bowl if desired.
Physical movement of slips by personnel is somewhat dangerous and time-consuming. Mechanical equipment to move the slips has also been utilized in the past to alleviate the manual handling. So far as is known, when such mechanically activated slips are disengaged from the pipe, the slips allowed the pipe to be run in and out of the hole, but would not open wide enough to allow the passage therethrough of oversized components with the pipe, such as packers and collars, without time-consuming disassembly of the slips. Thus, prior slip designs have had undesirable limitations.
Slips are also used in hydraulic workover units and snubbing rigs for well service operations. Workover rigs are portable, light weight and are generally used to control the injection and removal of tubing in a well. A snubbing unit is a special kind of workover rig which is suited to well workover where the well is under pressure. Snubbing units control negative and positive forces on the pipe as it is lowered or removed from the well which is under pressure. Because a snubbing unit is capable of handling tubing subject to forces in both axial directions, it typically utilizes four sets of slips, two of which grip and hold the pipe in each axial direction.